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I used to think scientists were being awfully carbon centric in their quest for life. There must be so many other ways for live to exist right? But once you t

I used to think scientists were being awfully carbon centric in their quest for life. There must be so many other ways for live to exist right? But once you take organic chemistry it seems so likely that other life would also be carbon based. Literally nothing else in the periodic table is as good as carbon for the flexibility that life benefits from so much. Silicon is a distant second. While it is certainly not impossible that there are forms of non-carbon based life out there, they would have to be utterly dwarfed by the number of carbon based ones.

rendall 3y ago
Our body of knowledge derives from intimate familiarity with an environment that is rare in the universe. The extremes here do not compare at all to cosmological extremes. It's not surprising that our chemistry is quite sophisticated and nuanced when the elements involved are abundant here and the temperatures involved are within a range easily achievable here and the gravity is more or less 1G and there is atmospheric pressure and the ambient magnetic field is inside a certain range and the time intervals involved are observable within a human attention span. I think we make a mistake by universalizing our perspective, so to speak. We define life by what we see around us and then assert, correctly and unassailably, that life must be made of the chemistry that we ourselves are made of, without generally understanding that the definitions are circular.
- jvanderbot 3y ago
  I'm lost. Presumably chemistry would work the same everywhere. So we should be able to recreate or predict some environments that would yield the same magic as 1g, 70deg F, carbon-based life, but with other parameters. Yet the consensus seems to be that there are relatively few other parameter sets that would yield some alien equivalent of organic chemistry. What gives?
  - rendall 3y ago
    It is very difficult for us humans to really grasp that unknown unknowns are real and have an effect. On some level, we assume that if we don't know something, it's not worth knowing, or we confidently make things up not even realizing we have done so. This bias influences us to believe that we know far more than we actually do. ...we should be able to recreate or predict some environments that would yield the same magic... Why should we? Physics and chemistry derive from observation. Theory is descriptive. 1) Our experience with chemistry outside of the Earth environment is almost entirely theoretical. Only very expensive equipment can achieve, for only fractions of a second, in only microliters volume, extremes of temperature, pressure, magnetic fields, and never any environment with higher gravity than our own. We have no experimental access to, not to mention actual intuitive understanding of, the widely diverse environments that exist in the Universe. So, our experimental knowledge is lacking. 2) Were we disembodied minds with no actual experience of the world, could we predict the existence of terrestrial life from first principles if given complete access to current knowledge of terrestrial physics and chemistry? Probably not. We don't know what makes a good environment for life, other than that we look around on Earth and see it everywhere. So, our theoretical knowledge is inadequate. 3) Even what we perceive and define as life, sentience or civilization is limited by our lack of experience. If trees for example had a rich storytelling tradition, epics unfolding over centuries perhaps, how would we even discover that or understand them? Trees are far more relatable than aliens are likely to be given that we share a common ancestor. The concept of, say, wind patterns or waves or nebulae being alive or sentient is easily dismissed, but again, our definition of life is narrowly defined by our single data point, our interests and outlook conferred and constrained by our evolution. Is it useful? Is it dangerous? This is the sort of thing that informs even our ability to perceive something as worthy of attention. Before microorganisms were discovered, the concept of invisible critters was laughable. So, our understanding of what we're looking for is inadequate. I suspect that the evidence of alien life is all around, but it's just too different even to be interesting.
- russfink 3y ago
  One hypothesis could be that life of all forms is abundant in the universe. If true, then that implies that our form of life is at least duplicated somewhere. If we find our form (or close cousin), then we open the way for the larger generalization.
boredemployee 3y ago
I wonder if in other places (galaxies and planets), the atoms could behave differently thus creating other forms of life. But I have no idea if that makes any sense.
- dr_hooo 3y ago
  From my understanding, there is a strong assumption backed by observations that matter and physics work the same way everywhere in our universe.
- finite_depth 3y ago
  Chemistry is somewhat environment- and temperature-dependent, but there's no other element that behaves like carbon in any known conditions. Carbon's chemistry comes from three major factors: (1) It forms four bonds readily. (2) It can form double- and triple-bonds readily. (3) It bonds strongly to itself in a configuration that allows it to form long chains. (1) is satisfied by other elements in its column on the periodic table, but silicon (the next element down in its group) and the following members (germanium, tin, and lead) fail (2) and increasingly (3). Silicon will form chains, but is reluctant to form double bonds; in general, double bonds become weaker for atoms further down the table. Silicon (and silicone, chains of Si-O bonds) are the most promising analogs but they have a lot of problems. (2) is satisfied by most other light nonmetals, but those nonmetals mostly fail (3) and almost all fail (1). The highly-electronegative oxygen doesn't really want to bond with itself (failing 3) and almost always takes a -2 oxidation state (failing 1). Nitrogen actually does form four-bond atoms decently often (most notably the ammonium ion), so it somewhat satisfies (1) to some extent, but is so eager to form N2 that most polynitrogen compounds are wildly unstable to the point that "nitro" is a term even laypeople know is associated with explosives (failing 3). Boron can form three bonds, allowing some of the complexity of (1), and will form nice polyboron compounds (3), but doesn't like forming double bonds (failing 2), and the bonds in boranes are so weak that they're mostly quite reactive (weakening 3). Sulfur will form polysulfur chains (the most common form is an eight-membered ring), but sulfur (like its cousin oxygen) usually doesn't form more than two bonds except with extremely electronegative partners (like its effective total bond order of 3 in sulfur dioxide or 4 in sulfur trioxide). ------ There's another problem here too: life is likely to form out of common elements in its environment, and carbon is just WAY more common than the alternatives. The mechanisms by which the elements are formed in stars very strongly favors elements with even atomic numbers (because they are mostly formed from helium-4 nuclei) and the burning processes peak at carbon/oxygen, neon, magnesium, and silicon. As a result, carbon is very common. It's the fourth most common element in the Universe (after hydrogen, helium, and oxygen), and ~an order of magnitude more common than any of the elements discussed above except oxygen (which has basically no analogs to carbon chemistry).
adastra22 3y ago
Silicon, germanium, titanium, and a few other elements can make structures as complex as carbon. But factor in relative abundances and solvating chemistry, and it starts looking very unlikely that life would evolve to be based on anything other than carbon.
- jacquesm 3y ago
  This fast. But given enough time life could evolve using other things than carbon, carbon is just way faster than anything else and would likely crowd out other contenders. But if for some reason a place is very low on carbon there is a chance for other chemistries to dominate even if their pathways are much slower than the ones in a carbon based environment. It used to be that the thinking was that this was simply unlikely to the point of being impossible but since 2017 there has been some change in attitude towards especially the possibility of silicon based life.
kragen 3y ago
this is an artifact of the room-temperature-centric history of experimentation, not some universal truth the humans live out their lives at 300°, absolute, using the kelvin scale. there's lots of interesting behavior in matter down to 4° or less, and some solids up past 4000°. going further, human science doesn't really have a good handle on what kinds of complex interactions can occur with alfvén waves and plasma currents; they don't even know what's going on with prominently (heh) observable phenomena on the surface of the sun or how to stabilize a tokamak. but to be conservative let's consider just the 4°–4000° temperature range where we have solids and liquids from that temperature range, covering three orders of magnitude, carbon-chain molecules have interesting and complex behavior over the range 200°–500°, more or less. just a factor of 2.5. 13% of the temperature range. below 200° (-73°C) pretty much all organic chemicals crystallize and become brittle and nonliving, though there are a few boring exceptions like methane. above 500° (230°C) they are unstable, breaking apart into elemental carbon and methane, again with a very few exceptions like teflon. the temperature range over which carbon chains are practical to synthesize is smaller still a chemist on titan (94° absolute) would consider implausible the suggestion that living cells could be made out of carbon-chain molecules dissolved in water, a material that doesn't even melt until hellish temperatures nearly three times the boiling point of methane. a chemist on venus (740° absolute) would consider implausible the suggestion that living cells could be made out of such absurdly unstable molecules as carbon-chain polymers — much less with amide groups, which she knows as violently reactive with ordinary everyday substances like red-hot sulfuric acid. moreover, she would consider the pressure on earth's surface (1% of atmospheric for a venusian) to be a good approximation to the vacuum of space, constantly exposed to sterilizing levels of ultraviolet from the sun (venus and titan specifically are not covered in vegetation or presumably in chemists, but there are astronomical numbers of other planets with conditions like theirs, and as sagan's paper pointed out, antarctica isn't exactly a jungle either) there are interesting things going on over the other 87% of the temperature range, too. at room temperature the humans mostly know phosphates as minerals that are unusually hard to melt, but phosphoric acid just starts to polymerize above about 500° absolute and stays liquid past 1000°, polymerizing more and more enthusiastically; some common phosphates remain solid past 2000°. neat silica becomes a useful solvent at temperatures a bit higher than that, and silicates of sodium or potassium can be liquid down to room temperature if plasticized with enough water; silica is nearly as abundant as carbon. ammonia is liquid from 195° to 240°, a narrower range than water (273° to 373°, which is a 3:4 ratio to ammonia's 4:5) but not ridiculously so, and shares many of water's interesting properties, and we already know many interesting substances that can be synthesized in an ammonia solution, including many that are not stable at room temperature (and therefore might be in a useful reactivity range in ammonia's liquid range). and there are 1.7 orders of magnitude of temperature below ammonia's freezing point before you get to liquid helium and the cosmic background radiation. most of the universe's volume is in that temperature range, though not most of its matter, and if dyson's 'future of life in an ever-expanding universe' is correct, those temperatures and below are where our descendants will spend the vast majority of history but the humans really only know a tiny fraction of the possible complex molecules at those temperatures, because working with them in their laboratories is difficult, expensive, dangerous, and unprofitable; molecules that explode when you heat them up to room temperature, or solidify into refractory ceramics when cooled anywhere near it, will not yield useful drugs, elastomers, cheap injection-molding plastics, or fuels, so research into them has mostly been pure, not applied. the exceptions are mostly high-temperature ceramics research focused on finding the strongest and most refractory materials, not interesting synthesis pathways that can operate inside white-hot zirconia crucibles but don't let the utterly disorderly behavior of, say, nitrogen chains at the melting point of water fool you into thinking their chemistry is trivial; most compounds that are stable at 10% of that temperature haven't been discovered yet (by the humans)
mnky9800n 3y ago
What is life? What motivates structures to possess mobility and self determination and all the other definitions? When is a self sustaining process something more than a summary of recent physical interactions? There are many self organising systems why are some life and others not? Stars seem to broadly fit most definitions of life. Why are they not considered alive? What is the distinction between living and nonliving allowing our understanding of the universe?
- s1artibartfast 3y ago
  Those are all really good questions, but at the end of the day they don't really matter and are a matter of semantics. You could similarly ask is there anything that isn't life. Is matter a prerequisite for life? What about voids in the matter of space that shift and change over time? Is empty space alive? You could Define life broadly to include Stars or mountains, but then you just look for a new subset and name that would include life more similar to our own. One definition of life that I think resists scrutiny is an object or unit which contains a transferable data and an algorithm to use that data. This would separate a cloud from a tree.
  - kragen 3y ago
    it doesn't distinguish you from certain reels of punched paper tape though
    - s1artibartfast 3y ago
      The viruses are considered to be alive, I don't see why tax forms wouldn't be alive either.
      - mangamadaiyan 3y ago
        I think the reference was to a Turing machine, in response to your mention of transferable data and algorithms.
tremon 3y ago
As I understood it, it would be perfectly possible to replicate most of the functionality of life (amino acids) by moving down one period in the periodic table. So that would be C->Si, N->P, P->As, etc (with or without O->S substitution [0]). You can't do the same by moving left or right in the periodic table because elements are grouped vertically by their electrochemical properties (electrovalence), so moving left or right gives you a material with wildly different properties. The problem is that all of the molecules in period 3 are much more massive than their carbon-family counterparts, which means that the energy required to construct said building blocks must be equally higher. If we assume a high-entropy environment exists for these molecules to form, the first obvious question is then: how come carbon-based life didn't appear in that environment as well and outcompeted Si-based life? Would it have to be high-pressure environments like inside gas giant planets, or high-temperature environments on planets much closer to the star? And if the environment is inhospitable to carbon-based life, is there sufficient stability in that environment to produce more than just the functional building blocks of life? Next, the question of development speed is an interesting one. Of course, evolution isn't linear so everything here is conjecture, but just as a thought experiment: How long would it take for such an environment (inhospitable to carbon-based life, but capable of spawning silicon-based life) to produce single-celled organisms, and then for those organisms to meaningfully alter their biosphere (like Earth's Great Oxidation Event) so that we can detect those changes through our telescopes? Earth is 4.5 billion years old; oxygen started appearing in the atmosphere 2 billion years ago, but didn't appear in meaningful quantities until less than 1 billion years ago. That 3.5 billion years from planet formation to detectable biosphere signature for carbon-based life (n=1, of course). If we take a wild guess that silicon-based life is slower to develop by a factor 2, we'd have to find planets that are at least 7 billion years old (and have been stable for that time). Given that the lower bound for the age of the entire universe is 8 billion years, we're getting close to planets that are as old as the universe itself. Add to that that our telescopes are also looking into the past, the further out we look -- but 100 million years (the Virgo supercluster) seems negligible compared to the supposed 7 billion years projected incubation time: It seems rather optimistic to expect silicon-based life signatures to readily appear on our telescopes. [0] and then there's also https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-water_solvents https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi... -- where they suggest NH3 and its ionized counterparts could serve as a replacement for H2O, OH- and H3O+

rendall 3y ago

Our body of knowledge derives from intimate familiarity with an environment that is rare in the universe. The extremes here do not compare at all to cosmological extremes. It's not surprising that our chemistry is quite sophisticated and nuanced when the elements involved are abundant here and the temperatures involved are within a range easily achievable here and the gravity is more or less 1G and there is atmospheric pressure and the ambient magnetic field is inside a certain range and the time intervals involved are observable within a human attention span. I think we make a mistake by universalizing our perspective, so to speak. We define life by what we see around us and then assert, correctly and unassailably, that life must be made of the chemistry that we ourselves are made of, without generally understanding that the definitions are circular.

jvanderbot 3y ago
I'm lost. Presumably chemistry would work the same everywhere. So we should be able to recreate or predict some environments that would yield the same magic as 1g, 70deg F, carbon-based life, but with other parameters. Yet the consensus seems to be that there are relatively few other parameter sets that would yield some alien equivalent of organic chemistry. What gives?
- rendall 3y ago
  It is very difficult for us humans to really grasp that unknown unknowns are real and have an effect. On some level, we assume that if we don't know something, it's not worth knowing, or we confidently make things up not even realizing we have done so. This bias influences us to believe that we know far more than we actually do. ...we should be able to recreate or predict some environments that would yield the same magic... Why should we? Physics and chemistry derive from observation. Theory is descriptive. 1) Our experience with chemistry outside of the Earth environment is almost entirely theoretical. Only very expensive equipment can achieve, for only fractions of a second, in only microliters volume, extremes of temperature, pressure, magnetic fields, and never any environment with higher gravity than our own. We have no experimental access to, not to mention actual intuitive understanding of, the widely diverse environments that exist in the Universe. So, our experimental knowledge is lacking. 2) Were we disembodied minds with no actual experience of the world, could we predict the existence of terrestrial life from first principles if given complete access to current knowledge of terrestrial physics and chemistry? Probably not. We don't know what makes a good environment for life, other than that we look around on Earth and see it everywhere. So, our theoretical knowledge is inadequate. 3) Even what we perceive and define as life, sentience or civilization is limited by our lack of experience. If trees for example had a rich storytelling tradition, epics unfolding over centuries perhaps, how would we even discover that or understand them? Trees are far more relatable than aliens are likely to be given that we share a common ancestor. The concept of, say, wind patterns or waves or nebulae being alive or sentient is easily dismissed, but again, our definition of life is narrowly defined by our single data point, our interests and outlook conferred and constrained by our evolution. Is it useful? Is it dangerous? This is the sort of thing that informs even our ability to perceive something as worthy of attention. Before microorganisms were discovered, the concept of invisible critters was laughable. So, our understanding of what we're looking for is inadequate. I suspect that the evidence of alien life is all around, but it's just too different even to be interesting.
russfink 3y ago
One hypothesis could be that life of all forms is abundant in the universe. If true, then that implies that our form of life is at least duplicated somewhere. If we find our form (or close cousin), then we open the way for the larger generalization.

boredemployee 3y ago

I wonder if in other places (galaxies and planets), the atoms could behave differently thus creating other forms of life. But I have no idea if that makes any sense.

dr_hooo 3y ago
From my understanding, there is a strong assumption backed by observations that matter and physics work the same way everywhere in our universe.
finite_depth 3y ago
Chemistry is somewhat environment- and temperature-dependent, but there's no other element that behaves like carbon in any known conditions. Carbon's chemistry comes from three major factors: (1) It forms four bonds readily. (2) It can form double- and triple-bonds readily. (3) It bonds strongly to itself in a configuration that allows it to form long chains. (1) is satisfied by other elements in its column on the periodic table, but silicon (the next element down in its group) and the following members (germanium, tin, and lead) fail (2) and increasingly (3). Silicon will form chains, but is reluctant to form double bonds; in general, double bonds become weaker for atoms further down the table. Silicon (and silicone, chains of Si-O bonds) are the most promising analogs but they have a lot of problems. (2) is satisfied by most other light nonmetals, but those nonmetals mostly fail (3) and almost all fail (1). The highly-electronegative oxygen doesn't really want to bond with itself (failing 3) and almost always takes a -2 oxidation state (failing 1). Nitrogen actually does form four-bond atoms decently often (most notably the ammonium ion), so it somewhat satisfies (1) to some extent, but is so eager to form N2 that most polynitrogen compounds are wildly unstable to the point that "nitro" is a term even laypeople know is associated with explosives (failing 3). Boron can form three bonds, allowing some of the complexity of (1), and will form nice polyboron compounds (3), but doesn't like forming double bonds (failing 2), and the bonds in boranes are so weak that they're mostly quite reactive (weakening 3). Sulfur will form polysulfur chains (the most common form is an eight-membered ring), but sulfur (like its cousin oxygen) usually doesn't form more than two bonds except with extremely electronegative partners (like its effective total bond order of 3 in sulfur dioxide or 4 in sulfur trioxide). ------ There's another problem here too: life is likely to form out of common elements in its environment, and carbon is just WAY more common than the alternatives. The mechanisms by which the elements are formed in stars very strongly favors elements with even atomic numbers (because they are mostly formed from helium-4 nuclei) and the burning processes peak at carbon/oxygen, neon, magnesium, and silicon. As a result, carbon is very common. It's the fourth most common element in the Universe (after hydrogen, helium, and oxygen), and ~an order of magnitude more common than any of the elements discussed above except oxygen (which has basically no analogs to carbon chemistry).

adastra22 3y ago

Silicon, germanium, titanium, and a few other elements can make structures as complex as carbon. But factor in relative abundances and solvating chemistry, and it starts looking very unlikely that life would evolve to be based on anything other than carbon.

jacquesm 3y ago
This fast. But given enough time life could evolve using other things than carbon, carbon is just way faster than anything else and would likely crowd out other contenders. But if for some reason a place is very low on carbon there is a chance for other chemistries to dominate even if their pathways are much slower than the ones in a carbon based environment. It used to be that the thinking was that this was simply unlikely to the point of being impossible but since 2017 there has been some change in attitude towards especially the possibility of silicon based life.

kragen 3y ago

this is an artifact of the room-temperature-centric history of experimentation, not some universal truth the humans live out their lives at 300°, absolute, using the kelvin scale. there's lots of interesting behavior in matter down to 4° or less, and some solids up past 4000°. going further, human science doesn't really have a good handle on what kinds of complex interactions can occur with alfvén waves and plasma currents; they don't even know what's going on with prominently (heh) observable phenomena on the surface of the sun or how to stabilize a tokamak. but to be conservative let's consider just the 4°–4000° temperature range where we have solids and liquids from that temperature range, covering three orders of magnitude, carbon-chain molecules have interesting and complex behavior over the range 200°–500°, more or less. just a factor of 2.5. 13% of the temperature range. below 200° (-73°C) pretty much all organic chemicals crystallize and become brittle and nonliving, though there are a few boring exceptions like methane. above 500° (230°C) they are unstable, breaking apart into elemental carbon and methane, again with a very few exceptions like teflon. the temperature range over which carbon chains are practical to synthesize is smaller still a chemist on titan (94° absolute) would consider implausible the suggestion that living cells could be made out of carbon-chain molecules dissolved in water, a material that doesn't even melt until hellish temperatures nearly three times the boiling point of methane. a chemist on venus (740° absolute) would consider implausible the suggestion that living cells could be made out of such absurdly unstable molecules as carbon-chain polymers — much less with amide groups, which she knows as violently reactive with ordinary everyday substances like red-hot sulfuric acid. moreover, she would consider the pressure on earth's surface (1% of atmospheric for a venusian) to be a good approximation to the vacuum of space, constantly exposed to sterilizing levels of ultraviolet from the sun (venus and titan specifically are not covered in vegetation or presumably in chemists, but there are astronomical numbers of other planets with conditions like theirs, and as sagan's paper pointed out, antarctica isn't exactly a jungle either) there are interesting things going on over the other 87% of the temperature range, too. at room temperature the humans mostly know phosphates as minerals that are unusually hard to melt, but phosphoric acid just starts to polymerize above about 500° absolute and stays liquid past 1000°, polymerizing more and more enthusiastically; some common phosphates remain solid past 2000°. neat silica becomes a useful solvent at temperatures a bit higher than that, and silicates of sodium or potassium can be liquid down to room temperature if plasticized with enough water; silica is nearly as abundant as carbon. ammonia is liquid from 195° to 240°, a narrower range than water (273° to 373°, which is a 3:4 ratio to ammonia's 4:5) but not ridiculously so, and shares many of water's interesting properties, and we already know many interesting substances that can be synthesized in an ammonia solution, including many that are not stable at room temperature (and therefore might be in a useful reactivity range in ammonia's liquid range). and there are 1.7 orders of magnitude of temperature below ammonia's freezing point before you get to liquid helium and the cosmic background radiation. most of the universe's volume is in that temperature range, though not most of its matter, and if dyson's 'future of life in an ever-expanding universe' is correct, those temperatures and below are where our descendants will spend the vast majority of history but the humans really only know a tiny fraction of the possible complex molecules at those temperatures, because working with them in their laboratories is difficult, expensive, dangerous, and unprofitable; molecules that explode when you heat them up to room temperature, or solidify into refractory ceramics when cooled anywhere near it, will not yield useful drugs, elastomers, cheap injection-molding plastics, or fuels, so research into them has mostly been pure, not applied. the exceptions are mostly high-temperature ceramics research focused on finding the strongest and most refractory materials, not interesting synthesis pathways that can operate inside white-hot zirconia crucibles but don't let the utterly disorderly behavior of, say, nitrogen chains at the melting point of water fool you into thinking their chemistry is trivial; most compounds that are stable at 10% of that temperature haven't been discovered yet (by the humans)

mnky9800n 3y ago

What is life? What motivates structures to possess mobility and self determination and all the other definitions? When is a self sustaining process something more than a summary of recent physical interactions? There are many self organising systems why are some life and others not? Stars seem to broadly fit most definitions of life. Why are they not considered alive? What is the distinction between living and nonliving allowing our understanding of the universe?

s1artibartfast 3y ago
Those are all really good questions, but at the end of the day they don't really matter and are a matter of semantics. You could similarly ask is there anything that isn't life. Is matter a prerequisite for life? What about voids in the matter of space that shift and change over time? Is empty space alive? You could Define life broadly to include Stars or mountains, but then you just look for a new subset and name that would include life more similar to our own. One definition of life that I think resists scrutiny is an object or unit which contains a transferable data and an algorithm to use that data. This would separate a cloud from a tree.
- kragen 3y ago
  it doesn't distinguish you from certain reels of punched paper tape though
  - s1artibartfast 3y ago
    The viruses are considered to be alive, I don't see why tax forms wouldn't be alive either.
    - mangamadaiyan 3y ago
      I think the reference was to a Turing machine, in response to your mention of transferable data and algorithms.

tremon 3y ago

As I understood it, it would be perfectly possible to replicate most of the functionality of life (amino acids) by moving down one period in the periodic table. So that would be C->Si, N->P, P->As, etc (with or without O->S substitution [0]). You can't do the same by moving left or right in the periodic table because elements are grouped vertically by their electrochemical properties (electrovalence), so moving left or right gives you a material with wildly different properties. The problem is that all of the molecules in period 3 are much more massive than their carbon-family counterparts, which means that the energy required to construct said building blocks must be equally higher. If we assume a high-entropy environment exists for these molecules to form, the first obvious question is then: how come carbon-based life didn't appear in that environment as well and outcompeted Si-based life? Would it have to be high-pressure environments like inside gas giant planets, or high-temperature environments on planets much closer to the star? And if the environment is inhospitable to carbon-based life, is there sufficient stability in that environment to produce more than just the functional building blocks of life? Next, the question of development speed is an interesting one. Of course, evolution isn't linear so everything here is conjecture, but just as a thought experiment: How long would it take for such an environment (inhospitable to carbon-based life, but capable of spawning silicon-based life) to produce single-celled organisms, and then for those organisms to meaningfully alter their biosphere (like Earth's Great Oxidation Event) so that we can detect those changes through our telescopes? Earth is 4.5 billion years old; oxygen started appearing in the atmosphere 2 billion years ago, but didn't appear in meaningful quantities until less than 1 billion years ago. That 3.5 billion years from planet formation to detectable biosphere signature for carbon-based life (n=1, of course). If we take a wild guess that silicon-based life is slower to develop by a factor 2, we'd have to find planets that are at least 7 billion years old (and have been stable for that time). Given that the lower bound for the age of the entire universe is 8 billion years, we're getting close to planets that are as old as the universe itself. Add to that that our telescopes are also looking into the past, the further out we look -- but 100 million years (the Virgo supercluster) seems negligible compared to the supposed 7 billion years projected incubation time: It seems rather optimistic to expect silicon-based life signatures to readily appear on our telescopes. [0] and then there's also https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-water_solvents https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi... -- where they suggest NH3 and its ionized counterparts could serve as a replacement for H2O, OH- and H3O+